This invention relates to circuits for electronic strobe lights such as are used to provide visual warning in electronic fire alarm devices and other emergency warning devices. These devices are frequently associated with audible warning devices such as horns, and provide an additional means for getting the attention of those persons who are in danger. For operation the strobe lights require a trigger circuit for initiating the firing of the flashtube. The trigger circuit can be considered part of the flash unit since its only use is to trigger the flash. Typically energy for the flash is supplied from a capacitor in shunt with the flash unit and occurs when the voltage across the flash unit exceeds the threshold value, typically 250 v., required to actuate the trigger circuit. After the flashtube is triggered, it becomes conductive and rapidly drains the stored energy from the shunt capacitor until the voltage across the flashtube has decreased to approximately 30 v. At that point, the flashtube extinguishes and becomes non-conductive.
Typical of the prior art devices is the circuit whose operation is shown in FIG. 1. This circuit, as shown in FIG. 1A, includes power supply terminals 2 and 4, across which is connected the supply voltage, and which may typically be 10/12 volts dc or 20/24 volts dc. Underwriters Laboratory specifications require that operation of the device must continue when the supply voltage drops to as much as 80% of the nominal value and also when it rises to 110% of the nominal value. Thus in the lower voltage range the unit must operate between 8 and 13.2 v., and in the upper voltage range operation must be sustained in the range of 16 v. and 26.4 v. It is also a requirement of UL specifications that the flash rates of such visual signalling devices must fall between 20 and 120 flashes per minute (FPM).
In FIG. 1A, the prior art device, an inductor L.sub.1 is connected by switch Q.sub.2 across the power supply to cause current I.sub.a to flow through the inductor and thereby store energy in it. Across the switch Q.sub.2 there is connected a series circuit comprised of a diode D.sub.5 and the parallel combination of a capacitor C.sub.4 and a flash unit 6. With the switch Q.sub.2 closed, as shown in FIG. 1A, no current will flow in the flash unit or capacitor C.sub.4.
When the switch Q.sub.2 is opened, as shown in FIG. 1B, the inductor, which was charged by the current flow I.sub.a will begin to discharge as its flux field collapses, and a current I.sub.b will flow through and charge the capacitor C.sub.4. In order to build up the voltage across the capacitor to the 250 v. needed to cause the flashtube to fire, when the power supply being used is a low voltage d.c., the switch is cycled at regular intervals. When the capacitor voltage has built up to 250 v., the flash unit will be fired to discharge the capacitor rapidly by the current flow I.sub.c, as shown in FIG. 1C, until the voltage across the capacitor drops to about 30 v. and the flashtube extinguishes. Strobe circuits, such as shown in FIG. 1, have been found to have a number of disadvantages. These include the disadvantage of having the capacitor charging current I.sub.b flowing in the lines from the supply. Such current flows may cause electromagnetic or radio frequency interference. This is particularly so in alarm installations which have long lead lines. Also, as is shown in FIG. 1C, the flash tube is effectively across the power line and the current I.sub.c is limited only by the effective d.c. resistance of the circuit and source, which is typically below in efficient designs. The result can then be a large destructive current.
Other problems which exist in the prior art devices include the tendency for the flash rate to vary sufficiently with variations in supply voltage to cause the flash rate to fall below or exceed the UL requirements. Also, it is desirable to have one unit which will operate with all of the normally encountered supply voltages.
In order to overcome these problems, it is an object of this invention to provide a strobe light circuit whose flash rate is not dependent on the supply voltage.
It is also an object of this invention to provide a unit which will operate over a range of 8 to 26.4 volts dc.
It is a further object of this invention to provide a strobe circuit whose configuration is such that there will be no excessive current in any stage of its operation.